Low molecular weight cellulose ethers are widely used in pharmaceutical applications or food, for example as a film-coating material on tablets, in pharmaceutical capsules or in light-colored food compositions.
A large variety of processes for producing low molecular weight cellulose ethers exist which start from a cellulose ether having a higher degree of polymerization and average molecular weight and which partially depolymerize the cellulose ether by acid catalyzed degradation, oxidative degradation or degradation by high-energy radiation or microorganism or enzymes.
A widely used method makes use of an acid for partial depolymerization. According to a process described in European Patent No. 210 917 the cellulose ether is depolymerized in essentially dry, powdery form, either by contacting it with dry hydrogen chloride gas or with an aqueous solution of hydrogen chloride.
U.S. Pat. No. 3,497,496 discloses a depolymerization process wherein a high-viscosity cellulose ether is dissolved in a solvent which essentially consists of an aliphatic carboxylic acid and sulfoacetic acid.
U.S. Patent Application Publication No. 2001/0020090 discloses an acid-catalyzed, hydrolytic depolymerization of an aqueous slurry of a cellulose ether. The water can be mixed with one or more organic suspension media, such as acetone, t-butanol and ethers. The cellulose ether is degraded in a special depolymerization apparatus equipped with a filter element to a product having a Hoeppler viscosity of not more than 50 mPa·s, measured in a 2 weight percent solution in water at 20° C.
A problem frequently observed in low molecular cellulose ethers is yellowing or discoloration. Such yellowing or discoloration can be observed in products into which the cellulose ethers are formulated. Yellowing or discoloration is easily observed in transparent pharmaceutical capsules and food products, which is highly undesirable. Several suggestions have been made to address this problem.
The International Patent Application WO 01/18062 discloses a process for acid-catalyzed depolymerization of cellulose ethers in a concentrated aqueous slurry in the optional presence of an oxidizing agent. The produced low molecular weight cellulose ethers are said to be of high purity and high whiteness.
U.S. Pat. No. 4,061,859 discloses a method of contacting a cellulose ether with gaseous HCl or another hydrogen halide gas and inhibiting the yellowing of the depolymerized product with sulfur dioxide gas. The cellulose ether can be contacted with the hydrogen halide gas as a dry powder or as a slurry in an organic solvent, such as methylene chloride, methanol, 1,1,1-trichloroethane, carbon tetrachloride, acetone, hexane or benzene. According to the examples the depolymerized product without sulfur dioxide treatment has a color of ranging from over 25 APHA to 125 APHA, measured as a 2 percent solution. The color is significantly reduced after sulfur dioxide treatment.
Since a low yellowing or discoloration of low molecular weight cellulose ethers is highly important, it would be desirable to provide another process for reducing the average molecular weight of a cellulose ether which results in a product with low yellowing or discoloration. It would be particularly desirable to provide a process which does not require the treatment with sulfur dioxide for reducing discoloration.